Depending on where the leak is, you might try fixing it yourself. I've done it using a torch and wheel weight lead but be sure to clean the area and use an acid flux for good flow. Neutalize the acid in the radiator after you have patched the leak by flushing.
I've also used JB Weld successfuly. I carry some in my fannypack for just such emergencies.
Another epoxy is a marine grade aluminum patch for boats. I've used this to repair hulls before and it should work on radiators too. It comes in a stick form and you use a propane torch and melt it on.
Always pressure check your system after affecting repairs to be sure you got it fixed before hitting the trail. If you are out on the trail and spring a leak, loosen the radiator cap to relieve the pressure and ride back easily to prevent boil over. An emergency stop leak is a handful of pepper. If you are out for a whole weekend and don't want to end the trip short, try it, it works!
Any radiator repair shop can give you an estimate to repair.